A single contact-type mechanical seal (hereinafter referred to as “Prior Art 1”) having a single sliding contact surface is a known example of a most commonly used conventional shaft-sealing device, as shown in FIG. 7. This single contact-type mechanical seal has a configuration in which a rotational seal element 2 is provided in a state that allows movement in the axial direction and allows integral rotation with a rotating shaft 1, a stationary seal element 4 is provided in a non-rotating state in the seal case 3, and these seal elements slide in close contact with each other along mutually opposing end faces in accordance with the operation of a spring 5 that urges the rotational seal element 2 in the axial direction.
However, when used as a shaft-sealing device for a high-pressure fluid, the single contact-type mechanical seal has a problem in that the load that acts on a sliding section 6 is large, abrasion of the sliding section 6 is considerable, and the length of time in which a good seal performance can be maintained is short. Also, since the fluid pressure inside a machine is greater than the pressure outside a machine, the fluid inside the machine is liable to leak outside the machine.
Also known is a double contact-type mechanical seal (hereinafter referred to as “Prior Art 2”) that has two sliding contact surfaces, as shown in FIG. 8. The double contact-type mechanical seal has a configuration in which two rotational seal elements 2 are provided facing outward in the axial direction in a state that allows integral rotation with a rotating shaft 1, two stationary seal elements 4 are provided in a non-rotating state in the seal case 3 and so as to face the two rotational seal elements 2, respectively, in a state that allows movement in the axial direction, and these seal elements slide in close contact with each other along mutually opposing end faces in accordance with the operation of a spring 5 that urges the stationary seal elements 4 in the axial direction. Also, a seal fluid under higher pressure than the pressure of the fluid inside the machine is introduced into the area between the two sliding sections 6 in order to prevent fluid inside the machine from leaking out.
In the double contact-type mechanical seal, the load that acts on the sliding sections 6 of the inner side of the machine is reduced by an amount equal to the pressure difference between the seal fluid and the fluid inside the machine, but the load that acts on the sliding sections 7 of the outer side of the machine is greater than that of Prior Art 1 because a seal fluid under higher pressure than the pressure of the fluid inside the machine is introduced into the area between the two sliding sections 6. As a result, there is a problem in that the abrasion of the sliding sections 7 is considerable and the length of time in which a good seal performance can be maintained is short. Also, there is a possibility that a large amount of seal fluid may be discharged to the exterior of the machine and such a configuration is less preferred.
Also known is a tandem contact-type mechanical seal (hereinafter referred to as “Prior Art 3”) that has two sliding contact surfaces and in which the seal of each sliding contact surface faces the same direction, as shown in FIG. 9. The tandem contact-type mechanical seal has a configuration in which two rotational seal elements 2 are provided in a state that allows integral rotation with a rotating shaft 1, two stationary seal elements 4 are provided in a non-rotating state in the seal case 3 and so as to face the two rotational seal elements 2 in the same direction in a state that allows movement in the axial direction, and these seal elements slide in close contact with each other along mutually opposing end faces in accordance with the operation of a spring 5 that urges the stationary seal elements 4 in the axial direction.
The tandem contact-type mechanical seal is generally used for high-pressure applications or for recovering fluid inside the machine, and in the case of a high-pressure application, the fluid pressure inside the machine is divided between the seal of the inner side the machine and the seal of the outer side the machine, and the load that acts on each sliding section 6 is reduced.
However, since the fluid pressure inside the machine is set to be highest, there is a possibility that carbon dioxide will leak into the atmospheric air in the case that liquid carbon dioxide is used as the fluid inside the machine, and the effect of wear of the sliding sections 6 is dramatic in the case of long-term use. Carbon dioxide that has leaked between the seal of the inner side of the machine and the seal of the outer side of the machine must be recovered.
Also known is a contact/contactless mechanism seal (hereinafter referred to as “Prior Art 4,” e.g., see Patent Document 1.) in which a contact-type mechanical seal and contactless mechanical seal are combined, as shown in FIG. 10. The contact/contactless mechanism seal has a contact-type mechanical seal, in which a rotational seal element 2 is provided in a state that allows integral rotation with a rotating shaft 1 of the outer side of the machine, a stationary seal element 4 is provided in a non-rotating state in the seal case 3 and in a state that allows movement in the axial direction, and these seal elements slide in close contact with each other along mutually opposing end faces in accordance with the operation of a spring 5 that urges the stationary seal element 4 in the axial direction. The contact/contactless mechanism seal also has a contactless mechanical seal in which a rotational seal element 7 is provided in a state that allows integral rotation with a rotating shaft 1 of the inner side of the machine, a stationary seal element 8 is provided in a non-rotating state in the seal case 3 and in a state that allows movement in the axial direction, and the mutually opposing end faces are kept by dynamic pressure so as not to be in contact with each other due.
In the contact/contactless mechanism seal, the load that acts on the seal part is low and a large pressure reduction can be produced by the contactless mechanical seal even when the fluid pressure inside the machine is high, because the contactless mechanical seal is disposed in the inner side of the machine. Therefore, the load that acts on the sliding section 6 of the contact-type mechanical seal disposed in the outer side of the machine can be reduced.
However, the contactless mechanical seal disposed in the inner side of the machine has a configuration in which the rotational seal element 7 and the stationary seal element 8 are slightly set apart by the dynamic pressure against the pressing force of the spring 5, a very small gap is formed between the end faces of the rotational seal element 7 and the stationary seal element 8, and a seal function is obtained while fluid inside the machine leaks into the gap. Therefore, fluid inside the machine fills the space between the two mechanical seals and there is a possibility that a portion of the fluid will leak into the atmospheric air from the contact-type mechanical seal of the outer side of the machine. This is a critical problem in a device that handles supercritical carbon dioxide. Also, the carbon dioxide that fills the space between the mechanical seal of the inner side of the machine and the mechanical seal of the outer side of the machine must be recovered.    Patent Document 1: Japanese Laid-open Patent Application No. 2002-98237